The present invention relates to a bidirectional linear switch and, more particularly, to a bidirectional linear MOS transistor switch suitable as a switch for use in an electronic exchange system.
A telephone exchange supplies a predetermined AC signal to a telephone line to be examined and measures an impedance of the load (telephone) when, for example, it is desired to measure the speech rate or to know the number of telephone sets installed. For a switch to control the supply of the AC signal, it is necessary that the linearity to the input AC signal of the characteristic of the switch in the conducting state (hereinafter, this linearity is referred to as a linearity of the ON characteristic) is good. In other words, it is required as the characteristic of the switch to have a predetermined constant impedance to the AC signal. As an example of such switches AC signals which are used in the conventional electronic exchanges, for example, there has been known a bidirectional switch which is constituted by two n channel type MOS transistors (hereinafter, referred to as nMOS) as disclosed in U.S. Pat. No. 4,170,740. In this switch, the substrate and source are short-circuited, respectively. However, the nMOS arranged in the direction opposite to the current flowing through the switch represents the parasitic diode characteristic due to the short-circuit between the substrate and the source, so that there is a problem that the linearity of the ON characteristic is lost.